Ask root for password when using su commandThat's it! Thanks it was commenting the line that says "auth suficient pam_root.so" ans adding the "auth required pam_unix.so". I don't know what exactlly the last command do, but was compulsory to get this work properly. Thanks a lot for your answers.

Ask root for password when using su commandMaybe this is useless. This is just a question from a lab in my university. I guess this is possible, because you can indicate a user the the commands he can use without password. For instance : gatoatigrado ALL=NOPASSWD: /bin/set-slow-cpufreq